1. Field
Embodiments of the present invention relate to image sensors and in particular, to lightly-doped drain (LDD) implants in image sensor transistors.
2. Discussion of Related Art
A conventional image sensor, which may be fabricated using a complementary metal oxide semiconductor (CMOS) technology or charge coupled device (CCD) technology, includes an array of pixels and associated readout circuitry. The pixel of a typical conventional CMOS image sensor may include a photosensitive element, a micro-lens, one or more light filters, a transfer transistor, a floating diffusion region, reset transistor, source follower, and a row select transistor.
The image sensor generally operates as follows. Light is incident on the micro-lens. The micro-lens focuses the light to the photosensitive element through the light filter. The photosensitive element converts the light into an electrical signal proportional to the intensity of the light detected. The transfer transistor is between the photosensitive element and the floating diffusion region and is used to transfer the electrical signal from the photosensitive element to the floating diffusion region. The electrical signal modulates the source follower. The reset transistor resets the photosensitive element. Keeping in mind that an image sensor typically includes an array of a large number of pixels, readout circuitry allows signals generated in the photosensitive elements to be processed into a usable image.
Conventional image sensors suffer from some limitations. As the pixel sizes of CMOS image sensors become smaller, for a greater density of pixels and lower cost, the active area of the photosensitive element becomes smaller. The smaller photosensitive element area can lead to lower sensitivity. If the photosensitive element has a relatively low sensitivity, the photosensitive element may not collect enough light. The result is that the reproduced image can be too dark and noisy. The smaller photosensitive element area also can lead to smaller full well capacity, which is the number of charges that can be held in the photosensitive element before it saturates.
Low sensitivity and reduced full well capacity can in turn lead to lower dynamic range and lower signal-to-noise ratio. Dynamic range is a measure of the photosensitive element's ability to adequately render both dark shadows and light in a scene.
One conventional method utilized to increase the area of the photosensitive element is to have multiple photosensitive elements share pixel transistors, such as the source follower, reset transistor, and row select transistor. For example, two combinations of photosensitive elements, transfer transistors, and floating diffusion regions can share one set of pixel transistors. Alternatively, four combinations of photosensitive elements, transfer transistors, and floating diffusion regions can share one set of pixel transistors. However, even with sharing, the pixel transistors still take up sizable space of the limited device area. For example, a 4-pixel-shared architecture may include 1.75 transistors per pixel. Alternatively, a 2-pixel-shared architecture may include 2.5 transistors per pixel.
Also, because the bias swings on the pixel transistors are high, the pixel transistors should be well isolated from each other and from the photosensitive elements. Shallow trench isolation (STI) typically performs this function.
Some conventional pixel transistors also have lightly-doped drain (LDD) source/drain implants. To account for process variations of the LDD implant photoresist pattern for both size and overlay, a minimum distance between the edge of the LDD defined by mask and photoresist and the pixel transistor source or drain should be maintained. A minimum distance should also be maintained between the photosensitive element and the LDD edge in order to prevent increased dark current and white pixels. If the design rules associated with these minimum distances are not adhered to, areas that should be implanted might not be implanted and areas that should not be implanted might accidentally be implanted. As a result of the design rules, the isolation distance between the photosensitive element and the source/drain of the pixel transistors may not be able to be made as small as desired.